Electroluminescent device



July 4, 1961 P. GOLDBERG ELECTROLUMINESCENT DEVICE Filed July 16, 19571N VEN TOR.

PAUL 60L DBERG A TTORNE Y My invention is directed towardelectroluminescent devices.

In the presence of an electric field, certain types of phosphors willluminesce, the intensity of the emitted light being a function of theelectric field intensity. Consequently, films or layers containing suchphosphors can be used to transform electrical energy to light energy.Phosphors of this type are said to be electroluminescent.

More particularly, one type of electroluminescent layer is formed from asuspension of electroluminescent powders in. dielectricmedia, asdescribed, for example, in the copending patent application Serial No.306,909, filed August 28, 1952, by Norman L. Harvey.

Conventionally an electroluminescent layer of this type is interposedand electrically connected between first and second electricallyconductive films at least one of which is transparent, thus forming anelectroluminescent device. A voltage is applied between the two films,and the device luminesces in accordance with the amplitude of an appliedalternating voltage. Such devices have been used, for example, as lamps,information storage and display devices and the like.

In the present state of the art, the luminescence of anyelectroluminescent device utilizing an electroluminescent layer of fixedgeometry and composition can only be varied by changing the ampliture orthe frequency of the applied voltage. In contradistinction, I havediscovered that the luminescence of such a device can be varied andindeed can be increased substantially over that hitherto obtainablewithout changing the frequency or amplitude of the applied voltage andfurther without changing the composition of the electroluminescentlayer.

Accordingly, it is an object of the present invention to increase theluminescence of an electroluminescent device without changing thefrequency or amplitude of the voltage applied thereto.

Another object is to provide a new and improved electroluminescentdevice of the character indicated.

Still another object is to increase the luminescence of anelectroluminescent device for a given applied voltage by optimizing thesize of the electroluminescent particles contained in theelectroluminescent layer.

These and other objects of the invention will either be explained orwill become apparent hereinafter.

The particles of electroluminescent phosphors conventionally used inelectroluminescent devices vary in size over a range of about 2 micronsto 40 microns. Surprisingly, I have discovered that when a voltagegradient having a predetermined value is established across anelectroluminescent layer having a fixed composition and geometry, i.e. afixed volume of phosphor in a predetermined volume of dielectric, thelight emission from the layer is dependent upon the mean particle sizeof the electroluminescent layer. Further, I have discovered that thisemission can be substantially increased over that hitherto obtainable bymatching or optimizing the mean particle size to correspond to thepredetermined value of the voltage gradient. Stated differently, thelight emission from an electroluminescent device having anelectroluminescent layer of given geometry and composition, for anygiven value of the voltage gradient established States Patent PatentedJuly 4, 1961 therein, will vary as the mean size of theelectroluminescent phosphor particles varies, and will attain maximumemission for one mean particle size corresponding to the given value ofthe voltage gradient. As the voltage gradi-.

ent increases, the mean size required for maximum emission decreases.

While it is not my intention to be bound by theory, I believe that thisrelationship between particle size and voltage gradient can be explainedas follows. For a given concentration of phosphor in suspendingdielectric and a given electroluminescent layer geometry, as the meanparticle size increases, a larger voltage drop appears across eachparticle. Since the brightness of any electroluminescent phosphorparticle increases as the voltage drop thereacross increases, eachparticle emits more light as the particle size increases. However, sincethe phosphor concentration is constant, as the particle size increases,the total number of particles decreases. These effects tend to opposeeach other. Hence, the relationship between light emission and particlesize for a given electroluminescent layer geometry and composition is afunction both of the particle size and the voltage gradient, and maximumlight emission is obtained when the particle size is matched with thevoltage gradient in the manner indicated above.

An illustrative embodiment of my invention will be explained withreference to the accompanying figure."

EXAMPLE I The mean particle size of a green electroluminescent phosphorof the type described in an article entitled Electroluminescent ZincSulfide Phosphors, published in the Journal of the ElectrochemicalSociety, vol. 100, pp. 566-57l (1953), (i.e. a zinc sulfide phosphoractivated with copper, chloride and trace amounts of lead) was measuredand found to be about 10 microns. A suspension of this phosphor incastor oil (phosphor concentration by volume of 25%) was prepared andplaced in a cell having a gap width of 5 mils. An alternating voltage offixed frequency (6000 c.p.s.) was applied across the cell causing thesuspension to lurninesce. The light emitted from a fixed area of thephosphor layer passed through a transparent side of the cell and wasmeasured photoelectrically. The amplitude of the alternating voltage wasvaried so as to respectively establish various voltage gradients acrossthe layer, and the light emitted was measured for each gradient in turn.

This phosphor was fractionated by settling through various liquids toproduce different samples having various mean particle sizes.Corresponding suspensions of these samples having the same phosphorconcentration as the unfractionated phosphor were tested in a cell underthe same conditions as above; i.e. the same gradients were establishedand the emitted light was measured in the same manner.

Illustrative results are tabulated in Table I below wherein thebrightness gain is the ratio of the brightness of a cell utilizing aspecified phosphor fraction to the brightness of a cell utilizing theunfractionated phosphor,

It will be apparent from a study of Table I that for each specifiedvoltage gradient there is one mean particle size that is optimum andresults in maximum brightness, and further that the optimum sizedecreases as the voltage gradient increases.

EXAMPLE II The procedure of Example I was repeated using a blueelectroluminescent phosphor having a mean particle size of 15 microns,different gradients and different particle sizes being used.Illustrative results are tabulated in Table 11 below.

Table II Mean Optimum Particle Size Field Strength, Volts/ cm.Brightness Gain It will be seen that again for each specified voltagegradient there is an optimum particle Size and that the optimum sizedecreases as the voltage gradient increases.

Further tests indicated that the above indicated relationships betweenvoltage gradient and particle size hold for all types ofelectroluminescent phosphor and were not dependent upon the frequency ofthe applied voltage.

While I have pointed out and shown my invention as applied above, itwill be apparent to those skilled in the art, that many modificationscan be made within the scope and sphere of my invention as defined inthe claim which follows.

What is claimed is:

An electroluminescent device comprising an electroluminescent layercontaining electroluminescent phosphor particles dispersed in dielectricmedia, opposite sides of said layer being coated with correspondingelectrically conductive films, at least one of said films being lighttransparent, and an alternating voltage source coupled between saidfilms to apply a voltage between said films to establish a voltagegradient of predetermined value in said layer, the mean particle size ofsaid particles being matched with said value, said mean size fallingwithin the approximate range 240 microns, the value of said gradientfalling within the approximate range 12,000-200,000 volts percentimeter, the mean sizes of said matched particles decreasing in anapproximately linear manner as the value of said gradient increases.

References Cited inthe file of this patent UNITED STATES PATENTS2,566,349 Mager Sept. 4, 1951 2,660,566 Froelich Nov. 24, 1953 2,728,870Gungle Dec. 27, 1955 2,819,420 Koller Jan. 7, 1958 2,824,992 BouchardFeb. 25, 1958

